Polyurethane (PUR) products frequently occur in industry, in particular in manufacturing and handling polyurethane foam, elastomers, adhesives and lacquers. Polyurethane is produced by the reaction of a bifunctional isocyanate with a polyfunctional alcohol. The satisfactory technical qualities of polyurethane have resulted in a large increase of its use and application fields during the last decade. In connection with thermal decomposition of polyurethanes, however, the formation of isocyanates, aminoisocyanates and amines might occur, and extremely high contents can be found in air, e.g. when welding automobile sheet steel. Besides the known types of isocyanate, also new types of aliphatic isocyanates have been detected, in connection with e.g. heat treatment of car paint. Most of the isocyanates formed have been found to be represented by so-called low molecular isocyanates. During short periods of heating, as is the case when welding, particularly high contents of isocyanates may be present (peak exposure). Of all the dangerous substances on the limit value list, isocyanates have the lowest permissible contents. Exposure to this new type of isocyanates was previously unheard of. Isocyanates in both gas and particle phase have been detected in connection with welding, grinding and cutting of painted automobile sheet steel, and respirable particles in high contents containing isocyanates have been detected. In thermal decomposition products of painted automobile sheet steel, detection has been made of, among other things, methyl isocyanate (MIC), ethyl isocyanate (EIC), propyl isocyanate (PIC), phenyl isocyanate (Phi), 1,6-hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 2,4- and 2,6-diisocyanate toluene (TDI) and 4,4-methylene diphenyldiisocyanate (MDI).
In thermal decomposition of phenol/formaldehyde/urea (FFU) plastic, isocyanic acid and methyl isocyanate are formed. FFU plastic is used, among other things, in wood glue and as a binder in mineral wool (and bakelite), which is frequently used as insulation for ovens and furnaces in industrial and domestic use. New fields of application in which exposure to isocyanates has been detected are the soldering and processing of printed circuit boards in the electronic industry, the welding, grinding and cutting of painted sheet steel in the automobile industry and the welding of lacquered copper pipes. Isocyanates have a varying degree of toxicity to the organism depending on their chemical and physical form. As a result, the hygienic limit values have been set at an extremely low level in all countries. For the exposed individual, the degree of exposure to isocyanates varies considerably in different operations during a working day and in connection with breakdowns. Thermal decomposition products from PUR constitute a special problem, since new and completely unknown isocyanates are formed, whose toxicity has not yet been analyzed in a satisfactory manner. Furthermore, the increasingly sophisticated measuring methods have revealed exposure to isocyanates in an increasing number of operations in industry.
To sum up, there are a number of operations in numerous working areas where people are daily exposed to or at risk being exposed to isocyanates at a varying degree. Considering the ominous tendency of isocyanates to cause respiratory diseases and the fact that there are some carcinogenic substances among the thermal decomposition products of polyurethane, e.g. 2,4-diamine toluene (TDA), 4,4-methylene diamine (MDA) and MOCA, it is very important to measure in a reliable, sensitive and rapid manner any presence of isocyanates, but also other decomposition products dangerous to health, in environments where there is such a risk.
Several commercially available direct reading instruments for isocyanates and other reactive organic and inorganic compounds are known. One type is based on a principle where e.g. air containing isocyanates is sucked through a cellulose filter paper. The filter paper (filter-tape) is impregnated with reagents. The result of the reaction between e.g. isocyanates and the reagents is that a color is formed (see references 1-7). The intensity of the color depends on the air concentration of e.g. isocyanates.
The measuring principle is based on a light source emitting light (of a certain wave length) to the impregnated filter tape. Then a detector measures the reflected light. The color formation with time on the impregnated filter is proportional with the air concentration of e.g. isocyanates. Such commercially available instruments are either static or dynamic. In the static instruments the impregnated filter is put on in the measuring device and the air concentration can be measured either by visual observation by comparison with a colored reference. In the dynamic measuring devices the tape is static for a certain period of time (typically 2 minutes) and thereafter the filter is moved and a new fresh filter is placed in the sucking zone.
These instruments typically provide continuous air monitoring of e.g. isocyanates and the air concentration can be instantaneously monitored. The measuring devices can be hand held or stationary. The measuring device needs to be calibrated for the compound that is to be measured. There may be interferences if several compounds are present in air.
Drawback with Current Techniques:
There are several drawbacks with the present types of instruments. One major drawback is that the response may vary if the compounds to be measured are present in particle form. Gas phase compounds will form a homogeneous color over the sampling spot on the filter whereas compounds in particle form will have a non homogenous color on the filter. Particles will create spots of more intense color on the filter. In addition droplets (particles) will impact on the filter resulting in incomplete reaction between the compounds to be measured and the reagent as the reagent and the compounds in the particles will not efficiently mix. The air levels will be underestimated. In fact, the principle does not correctly work for particle borne compounds.
The measurements of compounds in gas phase are basically more relevant, but still considerable limitations are present. If the air contains particles (such as soot and smoke) the particles will deposit on the filter. The measuring principle has the drawback that reflected light will be affected also by other compounds/particles that are deposited on the filter. Further, some isocyanates in the gas phase, such as methyl isocyanate (CH3NCO) and isocyanic acid (HNCO), will pass through the filter (breakthrough) and will therefore not react with the reagent and form a color and will not be detected (see references 1 and 2). Another drawback is that the color reaction will not only take place on the surface of the filter but also on deeper layers (and the backside) but it is only the color on the surface that is measured (detection losses). In addition, the reaction depends on the air humidity. If the air is very dry, the formation of color will be slow and much less color is formed as compared to humid air. Sufficient air humidity is critical for correct estimation of air concentration (see reference 1). The color formed for low molecular weight isocyanates with the old technique is not permanent which results in an unsatisfactory low sensitivity.
Further, it would be of interest to detect smaller hazardous compounds, e.g. smaller isocyanates, in a reliable and also quicker way than hereto known.
In view of this, there is a great demand for an improved sampling device and an improved method for sampling of products dangerous to health, such as isocyanates, aminoisocyanates, amines, and isothiocyanates, in a rapid, reliable, precise and tamper proof manner.